Silicon-on-insulator (SOI) technology has made great strides in recent years, and may possibly replace bulk silicon as the technology of choice for future VLSI circuits. SOI technology has dielectric isolation, rather than the twin tubs of conventional CMOS, and makes it practical to use the isolated silicon island as a circuit element.
The most commonly used EEPROMS use a floating gate and control gate (word line) elements in combination with programming either by hot-electron injection or by Fowler-Nordheim tunneling through a thin dielectric, and erasing by Fowler-Nordheim tunneling.
EEPROM cells may be fabricated using a standard CMOS process on bulk silicon without any additional processes. Such is described in a publication by K. Ohsaki et al., IEEE Journal of Solid State Circuits, Vol. 29, No. 3, p. 311, March 1994 entitled "A Single Poly EEPROM Cell Structure for Use in Standard CMOS Processes". The EEPROM cell consists of adjacently placed NMOS and PMOS transistors. The EEPROM cell uses only a single polysilicon layer which is patterned to provide a common polysilicon gate with respect to the NMOS and PMOS transistors. This polysilicon gate serves as the floating gate of the EEPROM cell. With bulk CMOS, this EEPROM implementation is very space consuming, requiring about 48 lithography squares, making the cell impractical for most applications.
The present state of the art in EEPROM design is represented, for example, in the publication by H. Kume et al. entitled "A 1.28 .mu.m.sup.2 Contactless Memory Cell Technology for a 3V-Only 64 Mbit EEPROM", in 1992 International Electron Devices Meeting, Technical Digest, p. 991. An EEPROM device consists of an n-channel field effect transistor having a floating gate of polysilicon and a control gate (word line) above the floating gate in a stack. The small cell area of 1.28 .mu.m.sup.2 is based on 0.4 .mu.m CMOS process (4 squares). The program/erase mechanism uses Fowler-Nordheim tunneling.
EEPROMs are useful for low power portable electronics and as microcodes for application specific integrated circuits (ASIC's) and for microprocessors.